1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a back-shooting type ink-jet printhead, in which two ink channels are provided symmetrically with respect to a nozzle, thereby improving a linearity of ejected ink droplets and increasing an operating frequency.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermal ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink to cause an ink droplet to be ejected due to the expansion force of the formed bubble. A second type is a piezoelectric ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink due to a deformation of a piezoelectric element.
An ink droplet ejection mechanism of a thermal ink-jet printhead will now be explained in detail. When a pulse current is supplied to a heater, which includes a heating resistor, the heater generates heat and ink near the heater is instantaneously heated to approximately 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated, and exert pressure on ink filling an ink chamber. As a result, ink around a nozzle is ejected from the ink chamber in the form of a droplet through the nozzle.
A thermal ink-jet printhead is classified into a top-shooting type, a side-shooting type, and a back-shooting type depending on a bubble growing direction and a droplet ejection direction. In a top-shooting type of printhead, a bubble grows in the same direction in which an ink droplet is ejected. In a side-shooting type of printhead, a bubble grows in a direction perpendicular to a direction in which an ink droplet is ejected. In a back-shooting type of printhead, a bubble grows in a direction opposite to a direction in which an ink droplet is ejected.
An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase a driving frequency.
FIG. 1 illustrates a partially exploded perspective view of a conventional top-shooting type ink-jet printhead. FIG. 2 illustrates a cross-sectional view of a vertical structure of the conventional ink-jet printhead of FIG. 1.
Referring to FIG. 1, the conventional ink-jet printhead includes a base plate 10, which is formed by stacking a plurality of material layers on a substrate, partition walls 20, which are stacked on the base plate 10 and define ink chambers 22, and a nozzle plate 30, which is stacked on the partition walls 20. Ink is contained in the ink chambers 22, and a heater 13, which is shown in FIG. 2, is disposed under the ink chambers 22 to heat the ink and generate bubbles. Ink paths 24 serve as paths through which the ink is supplied into the ink chambers 22 and provide flow communication from an ink container (not shown). A plurality of nozzles 32 is formed in the nozzle plate 30 at positions corresponding to the ink chambers 22 and allow the ink to be ejected therethrough.
The vertical structure of the ink-jet printhead will be explained with reference to FIG. 2. An insulation layer 12 is formed on a silicon substrate 11 to provide insulation between the heater 13 and the substrate 11. The heater 13 is formed on the insulation layer 12 to heat the ink filling the ink chambers 22 and generate bubbles. The heater 13 is formed by depositing a tantalum nitride layer or a tantalum-aluminum alloy layer on the insulation layer 12. A conductor 14 is disposed on the heater 13 to apply a current to the heater 13. The conductor 14 is made of a material having high conductivity, such as aluminum (Al) or an aluminum alloy.
A passivation layer 15 is formed on the heater 13 and the conductor 14 to protect the heater 13 and the conductor 14. The passivation layer 15 protects the heater 13 and the conductor 14 from being oxidized or directly contacting the ink, and is mainly formed by depositing a silicon nitride layer. Anti-cavitation layers 16 are formed on the passivation layer 15 at positions corresponding to the ink chambers 22.
The partition walls 20 are stacked on the base plate 10, which is formed by stacking the plurality of material layers, in order to define the ink chambers 22. The nozzle plate 30, in which the plurality of nozzles 32 is formed, is stacked on the partition walls 20.
In the ink-jet printhead constructed as above, the anti-cavitation layers 16 formed on the passivation layer 15 protect the heater 13 by preventing a cavitation pressure, which is generated when the bubbles burst, from being focused on a central portion of the heater 13. However, because of the anti-cavitation layers 16 formed on the passivation layer 15, the number of printhead manufacturing processes increases and it is difficult to transfer a sufficient amount of heat to the ink from the heater 15.
Recently, efforts have been made to increase the life span of the heater by making the ink paths asymmetric so that the cavitation pressure can be formed at regions other than the location of the heater or by distributing the cavitation pressure over a larger area so that the cavitation pressure can be decentralized.
FIG. 3 schematically illustrates a plan view of another conventional ink-jet printhead. Referring to FIG. 3, a heater 50 and a nozzle 52 are asymmetric with respect to a central portion of an ink chamber 54. An ink path 56 functions as a path through which ink is supplied into the ink chamber 54.
The conventional ink-jet printhead of FIG. 3 has advantages of changing the flow direction of the ink contained in the ink chamber 54 and reducing damage to the heater 50 caused when bubbles burst. However, the conventional ink-jet printhead in which the heater 50 and the nozzle 52 are asymmetric has disadvantages in that a linearity of ink droplets ejected through the nozzle 52 deteriorates, and a fluid that makes it difficult to refill the ink chamber 54 is generated, thereby decreasing an operating frequency of the printhead.